Secretion of the Adipocyte-Specific Secretory Protein Adiponectin Critically Depends on Thiol-Mediated Protein Retention

Abstract

Adiponectin is a secretory protein abundantly secreted from adipocytes. It assembles into a number of different higher-order complexes. Adipocytes maintain tight control over circulating plasma levels, suggesting the existence of a complex, highly regulated biosynthetic pathway. However, the critical mediators of adiponectin maturation within the secretory pathway have not been elucidated. Previously, we found that a significant portion of de novo-synthesized adiponectin is not secreted and retained in adipocytes. Here, we show that there is an abundant pool of properly folded adiponectin in the secretory pathway that is retained through thiol-mediated retention, as judged by the release of adiponectin in response to treatment of adipocytes with reducing agents. Adiponectin is covalently bound to the ER chaperone ERp44. An adiponectin mutant lacking cysteine 39 fails to stably interact with ERp44, demonstrating that this residue is the primary site mediating the covalent interaction. Another ER chaperone, Ero1-L␣, plays a critical role in the release of adiponectin from ERp44. Levels of both of these proteins are highly regulated in adipocytes and are influenced by the metabolic state of the cell. While less critical for the secretion of trimers, these chaperones play a major role in the assembly of higher-order adiponectin complexes. Our data highlight the importance of posttranslational events controlling adiponectin levels and the release of adiponectin from adipocytes. One mechanism for increasing circulating levels of specific adiponectin complexes by peroxisome proliferator-activated receptor gamma agonists may be selective upregulation of rate-limiting chaperones.Adiponectin, a secretory protein expressed at high levels in plasma, plays a beneficial role in diabetes and cardiovascular disease (5,21,25). Recently, adiponectin has been implicated as a critical mediator of peroxisome proliferator-activated receptor gamma (PPAR␥) agonist-induced insulin sensitization (13,22,27). The receptors for adiponectin that may mediate some aspects of adiponectin signaling in liver and muscle were recently described (20,46). The 247-amino-acid mouse adiponectin contains three domains, which include a short signal sequence followed by a hypervariable region, an N-terminal collagenous domain, and a C-terminal globular domain (30). Circulating adiponectin forms at least three complexes, which are referred to as the high-molecular-weight (HMW), lowmolecular-weight (LMW), and trimeric complexes (30, 43). Trimeric adiponectin is formed by interactions within the globular domain, further stabilized by a collagenous coiled coil that also supports formation of disulfide bonds between two of the three subunits mediated through cysteine 39 (Cys39). Different forms of adiponectin exert distinct functions. Absolute HMW levels and ratios of HMW to total levels of adiponectin in circulation change under different metabolic conditions (23,31,43,45). HMW levels are regulated in diabetic and cardiovascular disease states with reduced HMW...

Treatment of Obese Diabetic Mice With a Heme Oxygenase Inducer Reduces Visceral and Subcutaneous Adiposity, Increases Adiponectin Levels, and Improves Insulin Sensitivity and Glucose Tolerance

“…Because PPAR␥ also increases HO-1 protein levels (21) and HO-1 is known as a chaperone protein, it is possible that one of the mechanisms by which HO-1 can increase adiponectin levels is through more efficient adiponectin stabilization and protection. This would confirm the report of Wang et al (47) that showed that the chaperone protein EroL increased adiponectin. We also have previously shown that upregulation of HO-1 protein in diabetic rats provided both cardio-protection and vascular protection against ROS (24).…”

“…PPAR␥ agonists are shown to induce both HO-1 (21) and the rate-limiting chaperone protein EroL (46,47). PPAR␥ agonist that increases adiponectin may do so by increasing the levels of EroL chaperone protein (46).…”

A disulfide-bond A oxidoreductase-like protein (DsbA-L) regulates adiponectin multimerization

“…Several proteins have recently been shown to regulate adiponectin trafficking and secretion (27)(28)(29). However, the mechanisms regulating adiponectin multimerization and secretion remain largely unknown.…”

Integration of hormonal and nutrient signals that regulate leptin synthesis and secretion

“…However, leptin does not appear to colocalize within Golgi (3), and Roh et al (67) suggested that, at least in adipocytes from young rats, separate leptin storage vesicles may exist. Like adiponectin (84), the endoplasmic reticulum (ER) may be a critical site for regulating leptin folding (it has one disulfide bond) and secretion. Future studies in which the expression or binding activity of potential chaperones is manipulated might help to explain the cellular trafficking of leptin with variations in nutritional or physiological status.…”